CORAL GROWTH AND CO2 VARIATIONS : BIOLOGICAL AND CLIMATOLOGICAL IMPLICATIONS.

Until recently, biological activity was considered to be controlled by environmental parameters such as temperature, nutrient concentrations, oceanic circulation, stratification and sealevel change. Nonetheless, there is one parameter which has undergone tremendous fluctuations during Earth’s history : atmospheric CO2. Several observations exist which suggest that biological activity could have significantly influenced the evolution of CO2 in the past and that these atmospheric CO2 variations, which are intimately associated with climatic change, have in turn influenced the evolution of marine organisms. To improve our understanding of biogeochemical processes, my Ph.D. work focused on the relationship between metabolism and CO2. The “model” organism chosen was Scleractinian coral, or symbiotic coral, one of the predominant coral-reef building organisms.

A multi disciplinary approach which associates ecophysiology, biology, biogeo and geochemistry, modelling and paleoenvironment was necessary. The strategy developed is a synthesis of modelling (the development of a simple evolutive mathematical model of organism growth) and experiments (cultures in aquaria under controlled and semi-controlled conditions).

A non linear relationship, depending on the prevailing CO2 conditions, has been observed between processes (photosynthesis, calcification and respiration), biomass (organic and inorganic) and CO2. Processes and growth are enhanced (around 70%) by a CO2 increase similar to the one which occured during the last glacial-interglacial transition. A doubling of pCO2 as predicted to occur by 2100 (750 ppm) induces a decrease in the rate of calcification and inorganic growth (60% on average) while photosynthesis can be enhanced for some species. These results could suggest that coral growth has been enhanced during the last deglaciation and illustrate a possible weakening of coral reefs in the future as a result of anthropogenic perturbation.

If corals produce 1% of the total oceanic organic carbon, they could represent more than 20% of the total oceanic inorganic carbon production. This inorganic production is of the same order of magnitude as the calcareous phytoplankton such as coccolithophorids, or calcareous zooplankton such as foraminifera, the predominant calcareous organisms of the open ocean. In addition, coral feedback on atmospheric CO2 is always positive (i.e. sources of CO2), and of one order of magnitude larger than that of the calcifying phytoplankton. Calcareous phytoplankton behave as a CO2 sink for atm CO2 but could reduce the sink due to non calcareous phytoplankton such as diatoms from 10 to 60%. The feedback effect of marine organisms on atm CO2 depends then, on both internal (photosynthesis over calcification ratio, organic over inorganic carbon ratio) and external (CO2) parameters. This effect depends on the prevailing CO2 conditions and has therefore been fluctuating through our geological times.

While quantifying the interactions between CO2 and coral metabolism, this study suggests that CO2 concentration is a driving force for the evolution of marine organisms, in particular the growth, distribution and species successions both at geological and seasonal scales. This work approach reveals also a new way for the use of proxies in paleoreconstructions.

Contact : amat@bbsr.edu
More information on web site under construction : http://bbsr.edu/Labs/co2lab/alex/alex.html